In recent years, use of a wireless power feeding device that wirelessly feeds electrical power from a power transmission side to a power reception side as a means for transmitting power is progressing.
This wireless power feeding device can also be used for supplying electrical power between equipment such as a factory and a moving body such as an AGV (Auto Guided Vehicle) or a robot used in this facility.
The moving bodies such as the AGV and the robot move along a structural surface such as a floor surface or a wall surface of the facility.
The moving body receives electrical power from a power transmission coil of a power transmission unit provided on the structural surface through a power receiving coil.
In this case, an overall length of the power transmission coil for supplying electrical power is set so that electrical power can be fed to a plurality of moving bodies simultaneously.
That is, electrical power can be fed to the plurality of moving bodies simultaneously by the power transmission coil opposing the power receiving coil of the moving body.
However, in a case of wireless feeding utilizing magnetic field resonance, an impedance of the power transmission coil varies depending on the number of moving bodies.
When magnetic field resonance is used, so-called impedance matching for optimizing the impedance is required.
However, when the number of moving bodies to which electrical power is fed varies as described above, with the impedance optimized for one moving body, the impedance changes when electrical power is fed to two moving bodies, and there is a problem that power transmission efficiency decreases.
Conversely, with the impedance optimized for the two moving bodies, the power transmission efficiency for one moving body is decreased.
Furthermore, similar problems arise in a case of three or more moving bodies.
In order to solve such a problem, Japanese Patent Application Laid-Open Publication No. 2015-122920 (Patent Document 1) proposes an impedance matching by making a winding ratio of a variable transformer, that is, the number of turns of a coil variable.
However, in a case of magnetic field resonance utilizing a high frequency in a MHz region, if the variable transformer is used as in Patent Document 1, there is a problem that core losses become large and transmission efficiency is lowered.
In addition, transformers are large in size, and there is a problem that it is difficult to apply the transformers to wireless power feeding devices of AGVs and robots, which are intended to be installed in facilities such as factories.
That is, when installing the wireless power feeding device in a facility such as a factory, it is required to reduce the installation volume due to problems such as interference with surrounding equipment, and therefore it is difficult to use a transformer that is large in size.
Furthermore, since transformers utilize physical windings, there is a problem that it is difficult to ensure accuracy, and is difficult to match the impedances strictly.